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COSMIC RAYS AND EARTH’S CLIMATE

HENRIK SVENSMARKDanish Space Research Institute, DK-2100 Copenhagen Ø, Denmark

Submitted: 13 August 1999;

Abstract. During the last solar cycle Earth’s cloud cover underwent a modulation in phase withthe cosmic ray flux. Assuming that there is a causal relationship between the two, it is expected andfound that Earth’s temperature follows more closely decade variations in cosmic ray flux than othersolar activity parameters. If the relationship is real the state of the Heliosphere affects Earth’s climate.

1. Introduction

The physical cause of climate variability is not know in detail. There are sev-eral physical factors that are believed to influence Earth’s climate. For example:(1) Orbital changes in Earth’s motion around the sun is believed to cause ice-ages. (2) Internal variability in the climate system, e. g. changes in atmosphericand ocean circulation. (3) Large volcanic eruptions, which are known to causea sudden cooling lasting 2–3 years. A period with high volcanic activity couldpotentially lead to a cooling of Earth. (4) Changes in concentration of greenhousegases. Due to burning of fossil fuel during the last 100 years there has been anincrease in atmospheric CO2 concentration from about 280 to 365 ppm. BecauseCO2 is a greenhouse gas that traps outgoing long wave radiation, and that the sur-face temperature has increased by approximately 0.7 C during the last 100 years,there is a worry that this increase is leading to a warmer climate. (5) Changes insolar activity, which will be discussed further in this paper. The relative importanceof the above different influences is not know very well.

It is obvious from historical records that the Sun has played an important rolein the climate of the Earth. For more than a hundred years there have been re-ports of an apparent connection between solar activity and Earth’s climate (Eddy,1976; Herman and Goldberg, 1978). William Herschel, a well known scientist inLondon suggested in 1801 that the price of wheat was directly controlled by thenumber of sunspots, based on his observation that less rain fell when there was fewsunspots. Since then many reports have indicated a link between solar activity andclimate. Solar activity is known for a long time back in time through a history ofatmospheric isotope levels produced by galactic cosmic rays (see also the articleof J. Beer in this book). Such records reveal a striking qualitative agreement be-tween cold and warm climatic periods and low and high solar activity during thelast 10,000 years (Eddy, 1976). Figure 1 shows the variation in 14C concentrationduring the last millennium. From year 1000–1300 AD solar activity was very high,which coincided with the warm medieval period. It was during this period that theVikings settled in Greenland. Solar activity decreased considerably after 1300 ADand a long cold period followed, called the little ice age. This climatic shift was

Space Science Reviews 93: 155–166, 2000.c 2000 Kluwer Academic Publishers. Printed in the Netherlands.

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156 HENRIK SVENSMARK

Figure 1. Change in 14C concentration during the last 1,000 years. The variation in the 14C concentra-tion is dominated by changes in solar activity. When solar is high the production of C14 is low, due tothe shielding effect of the solar wind against cosmic rays. Note that the axis for the 14C concentrationhas been reversed. The Maunder minimum refers to the period 1645–1715 when very few sunspotswhere observed on the sun. In this period the concentration of 14C was higher in agreement with alow solar activity.

a disaster for the Vikings. The Little Ice Age lasted until the middle of the lastcentury. During this century solar activity has again increased and is at its highestlevel the past 600 years.

It is not only proxy data that show an apparent agreement with solar activity.An indication of a link between long term variations in solar activity and Earth’stemperature was found by Friis-Christensen and Lassen (1991) and Lassen andFriis-Christensen (1995). They showed that an empirically constructed measure ofsolar activity, the filtered solar cycle length, closely matched variations in northernhemispheric temperature during the last 400 years. Another interesting example ofa solar influence was discovered by Labitzke and van Loon (1993). They showedthat the height of the pressure surfaces in the lower stratosphere varies in phasewith solar activity and has done so through the last four solar cycles.

The most obvious and direct way solar activity could affect Earth’s climatewould be via changes in solar irradiance. But the steadiness of the sun has beenestablished by satellite measurements of solar irradiance during the last 20 years.It is found that the variations are small (0.1% 0.3 W/m2 during a solar cycle),although this is not completely negligible, it is too small to explain the observedtemperature changes (Lean et al., 1995). Apart from a direct influence of solarirradiance variations, there have been speculations on how small changes in solaractivity can be amplified in the Earth’s atmosphere. One idea is related to the factthat during a solar cycle, changes in the UV radiation of the solar spectrum areof the order of 10% . Due to the importance of UV in the formation of ozone it


COSMIC RAYS AND EARTH’S CLIMATE 157

has been suggested that the resulting heating in the stratosphere is dynamicallytransported down into the troposphere (Haigh, 1996; Shindell et al., 1999).

Another suggestion involves galactic cosmic rays (GCR). GCR consists of veryenergetic particles (mainly protons) that are accelerated in stellar processes in ourgalaxy. Some of them enter Earth’s atmosphere where nuclear processes take placeand produce secondary particles that can penetrate still deeper into the atmosphere(Lal et al., 1967). Ionisation in the atmosphere below 35 km is almost exclusivelyproduced by GCR, except for the lowest 1 km over land where radioactive gasesare the main cause of ionisation. Ionisation by GCR is the variable of the loweratmosphere subject to the largest solar cycle modulation(Ney, 1959). The ionisa-tion variation could potentially influence optical transparency of the atmosphere,by either a change in aerosol formation and/or an influence on the transition be-tween the different phases of water (Ney, 1959; Dickinson, 1975; Pudovkin andRaspopov, 1992; Pudovkin and Veretenenko, 1992; Tinsley, 1996; Svensmark etal., 1997; Svensmark et al., 1998).

In the following it will be shown that Earth’s cloud cover, obtained from satel-lites, within the last solar cycle follows variations in GCR more closely than othersolar activity parameters. Further it will be shown that long term variation in solaractivities given by GCR reflects variations in Earth’s temperature during the period(1937–1994) based on direct measurement of cosmic ray flux. Finally an indicationof a GCR influence on temperatures during the Maunder minimum is presented.

2. Cosmic Rays and Earth’s Climate

2.1. COSMIC RAYS AND CLOUDS

Recently it was found that the Earth’s cloud cover, observed by satellites, is stronglycorrelated with GCR (Svensmark et al., 1997). Clouds influence vertically inte-grated radiative properties of the atmosphere by both cooling through reflectionof incoming shortwave radiation, and heating through trapping of outgoing long-wave radiation. The net radiative impact of a particular cloud is mainly dependentupon its height above the surface and its optical thickness. High optically thinclouds tend to heat while low optically thick clouds tend to cool (Hartmann, 1993).With a current climatic estimate for the net forcing of the global cloud cover asa 17–35 Wm 2 cooling, clouds play an important role in the Earth’s radiationbudget (Ohring and Clapp, 1980; Ramanathan et al., 1989; Ardanuy et al., 1991).Thus any significant solar influence on global cloud properties is potentially veryimportant for Earths climate (Svensmark et al., 1997; Svensmark et al., 1998).

Figure 2 is a composite of satellite observations of Earth’s total cloud coveradapted from Svensmark et al. (1997). The cloud data comprise the NIMBUS-7 CMATRIX project (Stowe et al., 1988) (triangles), and secondly the Interna-tional Satellite Cloud Climatology Project (ISCCP) (Rossow and Schiffer, 1991)(squares). Finally data from the Defense Satellite Meteorological Program (DMSP)



Figure 2. Composite figure showing changes in Earth’s cloud cover from four satellite cloud data setstogether with cosmic rays fluxes from Climax (solid curve, normalized to May 1965), and 10.7 cmSolar flux (broken curve, in units of 10 22Wm 2Hz 1). Triangles are the Nimbus7 data, squares arethe ISCCP_C2 and ISCCP_D2 data, diamonds are the DMSP data. All the displayed data have beensmoothed using a 12 months running mean. The Nimbus7 and the DMSP data are total cloud coverfor the Southern Hemisphere over oceans, and the ISCCP data have been derived from geostationarysatellites over oceans with the tropics excluded.

Special Sensor Microwave/Imager (SSM/I) (diamonds) (Weng and Grody, 1994).Since the cloud data are obtained from four different cloud satellite programs thedata presented are not homogeneous. The reason is that the instrumentation, spatialand temporal coverage is different from one satellite system to another. Thereforeonly the relative variations in the data can be compared. Only the most reliabledata common to all satellites was used in an attempt to improve inhomogeneities.For example the DMSP satellites only retrieve data from over the oceans, there-fore only cloud retrieved over oceans is used. Further, only ISCCP data retrievedfrom geo-stationary satellites was used due to their superior spatial and temporalcoverage over the polar orbiting satellites. The tropics was excluded in the aboveFig. 2 for two reasons. Due to the shielding of Earth’s magnetic field there is asignificant reduction of GCR flux close to the equator. Secondly, tropical cloudprocesses there are different compared to cloud processes at higher latitudes, e. g.the net radiative impact of clouds in the tropical regions is small compared to higherlatitudes. For further details see Svensmark et al. (1997). The obtained results arenot very sensitive to the selection mentioned above, and therefore the term Earth’scloud cover is used in Fig. 2.

In the figure the cloud data is compared with variation in GCR and the 10.7 cmradio flux from the Sun. One sees that there are clear differences between thevariation of GCR and the radio flux. From 1987 to present the two follow eachother. However, there is a lag between the two of almost two years prior to 1987.What is crucial in this context is that Earth’s cloud cover follows the variation in



Figure 3. Top curve is cosmic ray flux from the neutron monitor in Climax, Colorado (1953–1996).Middle curve is annual mean variation in Cosmic Ray flux as measured by ionisation chambers(1937–1994). The neutron data has been normalized to May 1965, and the ionisation chamber datahas been normalized to 1965. Bottom curve is the relative sunspot number.

GCR. This is important since it indicates that it is the ionisation in the atmosphereproduced by GCR that is essential in the solar climate link, and not necessarily thevariations in the 10.7 cm flux. This radio flux follows closely variations in totalsolar irradiance, soft X-rays, and in ultraviolet radiation.

2.2. INFLUENCE ON TEMPERATURE

Having established that variations in GCR are a good candidate for indirectly in-fluencing Earth’s climate based on data covering the last solar cycle, it is importantto compare variations in solar activity over a longer time span. However, there isno reliable data of cloud cover outside the period already used. But if variationsin GCR cause a climatic effect it should be reflected in variations in Earth’s tem-perature. To investigate this a long data series of GCR flux is needed. Instrumentalrecordings of cosmic rays started around 1935. The first measurements where per-formed primarily with ionisation chambers, which measure mainly the muon flux.Muons are responsible for most of the ionisation in the lower part of the tropo-sphere (Lal et al., 1967). Ahluwalia has constructed a measure of cosmic ray flux,based on ion chambers, covering the period 1937 to 1994 (Ahluwalia, 1997), whichis shown in Fig. 3. This extended data string is made by combining annually meanhourly counting rates from Cheltenham/Fredericksburg (1937–1975) and Yakutsk(1953–1994). These data represent part of the high energy GCR spectrum, andis only modulated about 4% during a solar cycle. Also contracted in Fig. 3 are



Figure 4. 11-year average of Northern Hemispheric marine and land air temperature anomalies (bro-ken line) compared with, a) unfiltered solar cycle length. b) 11-year (box-car) average of cosmicray flux (from ion chambers 1937–1994, normalized to 1965, thick solid line), the thin solid lineis cosmic ray flux from Climax, Colorado neutron monitor (arbitrarily scale), c) 11-year (box-car)average of relative sunspot number, d) decade variation in reconstructed solar irradiance (zero levelcorrespond to 1367 W/m2, adapted from Lean et al., 1995).

data from the Climax neutron monitor (1953–1995) in Colorado, which measuresthe low energy nucleonic part of the GCR spectrum. For comparison the relativesunspot number is plotted, which follows closely the solar 10.7 cm flux. Note thateven though there is a clear solar cycle modulation of the Cosmic ray flux, the min-imum in GCR and are not well correlated with the maximums in sunspot number(Ahluwalia, 1996), which gives a possibility to make a distinction between longterm trends in the two.

Figure 4 displays four different measures of long term solar activity togetherwith Earth’s temperature. (The temperatures used here and in the following are alltemperature anomalies). In the figure, 11-year averages of the northern hemisphericland and marine temperatures (Jones, 1997) are shown in all four panels. The north-ern hemispheric temperatures are chosen for two reasons, first the data are superiorsince there are far more recordings compared to the southern hemisphere. Secondlythe southern hemisphere is mainly water and the thermal inertia of the ocean tendsto mask a solar forcing in contrast to the northern hemisphere. Panel 4a shows in



addition the unfiltered solar cycle length. Panel 4b displays the 11-year averaged(ion chamber 1937–1994) cosmic ray flux (thick solid line). For comparison theClimax neutron monitor is also shown (thin solid line, scale not shown). Panel 4cshows the 11-years average of sunspot number, and finally panel 4d is decadevariations in reconstructed solar irradiance adapted from Lean et al. (1995). Thebest correspondence between solar activity and temperature seems to be betweensolar cycle length and variations in cosmic ray flux. However, the closest match iswith the ion chamber cosmic ray data. This is interesting since these "high energy"cosmic rays are responsible for ionisation in the lowest part of the atmosphere(below about 4km), and might hint at where in the atmosphere to look for a physicaleffect. The variations in reconstructed solar irradiance follows more closely thevariations in the sunspot number panel 4d.

From Fig. 4 it is seen that the temperature in the period 1970–1990 rose byapproximately 0.3 C. It is possible to compare the variation in cosmic ray fluxand this temperature change via some simple assumptions. From cloud satelliteobservations and numerical cloud modeling it is found that a 1% change in the totalcomposition of Earth’s cloud cover correspond to 0.5 W/m2 change in net radiativeforcing (Rossow and Cairns, 1995). From Svensmark et al. (1997) it is knownthat from 1987 to 1990 global cloudiness changed approximately 3.0% which canbe estimated to 1.50 W/m2. In the same period cosmic rays from the ion chamberchanged 3.5% as seen in Fig. 3. We can now calculate the approximate radiativeforcing by noting that the running mean 11-years average increase of cosmic raysin Fig. 4 from 1975 to 1989 is between 0.6–1.2% which then corresponds to apotential 0.3–0.5 W/m2 change in cloud forcing. This is a fairly large forcing, about2–4 times the estimated change in solar irradiance. Studies obtained from a generalcirculation model gave a sensitivity (0.7 to 1 C/Wm 2 for S 0 25%, where Sis the solar constant) (Rind and Overpeck, 1993). The direct influence of changesin solar irradiance is estimated to be only 0.1 C (Lean et al., 1995). The cloudforcing however, gives for the above sensitivity, 0.2–0.5 C. The basic assumptionin the above simple calculation is that the whole cloud volume is affected by solaractivity. This is consistent with the result from Fig. 4 that shows that an increasein cloud cover results in lower temperatures. Solar forcing therefore has the poten-tial to explain a significant part of the temperature changes over the period studied.

2.3. MAUNDER MINIMUM

The Maunder minimum (MM) (1645–1715) is a famous period in the Sun’s history.It is a period where very few sunspots where observed. In 1976 Eddy suggestedthat during this period the solar output was lower, and that this reduction couldexplain the extreme climatic conditions (Eddy, 1976) at the time. Since then var-ious efforts have gone into reconstructing solar irradiance back in time. One ofthe better measures of solar activity is the sunspot number, which is known overthe last 400 years. Therefore it is tempting to look for a relation between sunspotnumber and changes in solar irradiance observed by satellites during the last 20



years, since such a relation can be used in an attempt to reconstruct solar irradi-ance back in time. Figure 5a shows part of a reconstructed solar irradiance curveconstructed by Lean et al. (Lean et al., 1995), centered around the MM. Assumingthat the sun is in a non-cyclic state during the MM, the irradiance is found to belower by 0.24% ( 0.82 W/m2 when averaged over Earth’s surface) lower than thepresent day value (Lean et al., 1992). As a result, the reconstructed solar irradianceshown in Fig. 5a is nearly con-stant during the whole period ofthe MM. Figure 5b shows thevariations in 10Be, and is a sig-nature of changes in cosmic rayflux during the MM (Beer etal., 1991). The 10Be data therebycontains information on varia-tions in the solar wind magneticactivity. It is seen that there seemto be a cyclic magnetic behav-ior through the MM (Beer et al.,1985), and that the very low so-lar magnetic activity is occur-ring at the end of the MM, i. e.1690–1715. It is interesting tocompare the above curves for so-lar irradiance and the 10Be data,i. e. Figs. 5a and b with a re-cently reconstructed temperaturecurve, Fig. 5c, for the north-ern hemisphere (Jones et al.,1998). A striking similarity be-tween the 10Be curve and thetemperature is seen. In fact thedecade 1690–1700 is the coldestduring the last 1000 years, at thesame time as the 10Be concentra-tion has the largest peak. Assum-ing there is a solar impact on cli-mate during this period, it seemsless likely climate was affectedby the Sun in a non-cyclic state,which gives the nearly constantsolar irradiance, according to the

Figure 5. Panel a): Variation in in reconstructed so-lar irradiance during the Maunder minimum, fromLean et al. Panel b): Variation in 10Be concentrationduring the Maunder minimum (note that the 10Beaxis is reversed). Panel c): Reconstructed tempera-ture anomalies of the northern hemisphere during theMaunder minimum.

model of Lean et al. Rather, the good agreement between temperature and 10Beconcentration could suggest that cosmic rays are important in a sun/climate link.



Alternatively it could be that the origin of the irradiance changes during the MMare not understood. Of course the above conclusions hinge on how accurate thedata are. The peak in the 10Be curve might in fact be broader (Beer, 2000), and thetrue shape of the minimum in the temperature curve could be different. Howeverreconstructed temperatures of the northern hemisphere, by Mann et al. (1998), alsofinds that the coldest decade is 1690–1700.

3. Conclusions

Climate has been varying during all time. The origin of these variations has pre-viously, and almost exclusively, been attributed to internal causes. For examplevolcanic dust in the stratosphere can cause cooling of the order 0.5 C for a year ormore. The same goes for the atmospheric/ocean oscillation in the Pacific called El

Southern Oscillation . So it is expected that the annual variations in temper-ature will be a composite of several causes, of which only one is a solar influence.However, at time scales greater than 10 years it looks like the Sun has a signif-icant influence on climate variations. This statement is based on the qualitativeagreement between isotopes and proxy data for Earth’s temperature over the last1000 years.

A remarkable correlation between cosmic ray flux and variations in Earths cloudcover has been demonstrated. Since clouds are important for the Earths energybalance, a solar influence on clouds could be the main cause for the observedcorrelations between the sun and Earths climate. However, as is well known agood correlation does not guarantee a physical cause and effect. It is thereforenecessary to get an understanding of the microphysical mechanism that connectssolar activity with Earth’s cloud cover. If the influence of cosmic rays on clouds isreal, then it is thought that ionisation produced by GCR affects the microphysicsin cloud formation. There is currently an initiative do an experiment at Europeancentre for particle physics CERN, in Geneva, to test this hypothesis. The idea is tostudy the effect of ionisation caused by cosmic rays on droplet formation. This willbe done in a cloud chamber with a high degree of control of all relevant param-eters. However, future efforts to understand the importance of solar activity mustalso involve observations. In particular it is important to find what type of cloudsare affected. This can be done on the one hand from further studies by satelliteobservation, but also by regional observations of cloud/aerosol formation, e.g. byLIDAR. It is hoped that the present study might increase the interest in finding aphysical mechanism.

Acknowledgements

I thank J.van der Plicht for providing the C14 data that Fig. 1 was based on. Alsothanks to J. Beer for providing 10Be data that Fig. 5b was based on.



References

Ahluwalia, H. S., and Wilson, M. D.: 1996, ‘Present Status of the Recovery Phase of Cosmic ray11–year Modulation’, J. Geophys. Res. 101, 4879–4883.

Ahluwalia, H. S.: 1997, ‘Galactic Cosmic Ray Intensity Variations at High Latitude sea Level Site1937–1994’, J. Geophys. Res. 102, 24,229–24,236.

Ardanuy, P., Stowe, L. L., Gruber, A., and Weiss, M.: 1991, ‘Shortwave, Longwave, and net Cloud-Radiative Forcing as Determined From Nimbus-7 Observations’, J. Geophys. Res. 96, 1–2.

Beer, J., et al.: 1985, ‘Accelerator Measurements of 10Be: The 11 Year Solar Cycle From 1180-1800’,Nucl. Instru. Meth. Phys. Res. B10/11, 415–418.

Beer, J., Raisbeck, G. M., and Yiou, F.: 1991, in C. P. Sonett, M.S. Giampapa, and M.S. Matthews(eds.), The Sun in Time, University of Arizona Press.

Beer, J., personal communication.Dickinson, R., 1975, ‘Solar Variability and the Lower Atmosphere’, Bull. Am. Met. Soc. 56, 1240–

1248.Eddy, J. A.: 1976, ‘The Maunder Minimum’, Science 192, 1189–1202.See for example the book: Herman, J. R., and Goldberg, R. A.: 1978, Sun, Weather, and Climate,

NASA SP 426.Friis-Christensen, E., and Lassen, K.: 1991, ‘Length of the Solar Cycle: An Indicator Closely

Associated With Climate’, Science 254, 698–700.Haigh, J. D.: 1996, ‘The Impact of Solar Variability on Climate’, Science 272, 981–984.Hartmann, D. L.: 1993, ‘Radiative Effects of Clouds on Climate’, in P. V. Hobbs (ed.), Aerosol–

Cloud–Climate Interactions, Academic Press, pp. 151–173.Jones, P. D.: 1997, ‘Hemispheric and Global Temperatures, 1851–199, Part 1’, Climate Monitor 25,

20–30; Jones, P. D.: 1997, ‘Hemispheric and Global Temperatures, 1851–1996, Part 2’, ClimateMonitor 25, 66–77.

Jones, P. D., Briffa, K. R., Barnett, T. P., and Tett, S.F.B.: 1998, ‘High-Resolution PalaeoclimaticRecords for the Last Millennium: Interpretation, Integration and Comparison With GeneralCirculation Model Control-run Temperatures’, The Holocene 8, 455–471.

Labitzke, K., and van Loon, H.: 1993, ‘Some Recent Studies of Probable Connections Between Solarand Atmospheric Variability’, Ann. Geophysicae 11, 1084–1094.

Lal, D., and Peters, B.: 1967, in S. Flugge (ed.), Encyclopedia of Physics, number XLVI in 2,Springer-Verlag, Berlin Heidelberg, p. 551.

Lassen, K., and Friis-Christensen, E.: ‘Variability of the Solar Cycle Length During the Last FiveCenturies and the Apparent Association with Terrestrial Climate’, J. atm. terr. Phys. 57, 835–845.

Lean, J., Skumanich, A., White, O.: 1992, ‘Estimating the Suns Radiative Output During theMaunder Minimum’, Geophys. Res. Lett. 19, 1591–1594.

Lean, J., Beer, J., and Breadley, R.: 1995, ‘Reconstruction of Solar Irradiance Since 1610:Implications for Climate Change’, Geophys. Res. Lett. 22, 3,195–3,198.

Mann, M. E., Bradley, R. S., and Huges, M. K.: 1998, ‘Global-Scale Temperature Patterns andClimate Forcing Over the Past six Centuries’, Nature 392, 779–787.

Ney, E. R.: 1959, ‘Cosmic Radiation and the Weather’, Nature 183, 451–452.Ohring, G., and Clapp, P. F.: 1980, ‘The Effect of Changes in Cloud Amount on the net Radiation at

the top of the Atmosphere’, J. Atmos. Sci. 37, 447–457.Pudovkin, M. I., and Raspopov, O. M.: 1992, ‘The Mechanism of Action of Solar Activity on the

State of the Lower Atmosphere and Metorological Parameters (A Review)’, Geomagn. andAeronomy 32, 593–608.

Pudovkin, M., and Veretenenko, S.: 1995, ‘Cloudiness decrease associated with Forbush Decreasesof Galatic Cosmic Rays’, J. Atmos. Terr. Phys. 57, 1349–1355.



Ramanathan, Cess, R. D., Harrison, E. F., Minnis, P., Barkstrom, B. R., Ahmad, E., and Hartmann,D.: 1989, ‘Cloud-radiative Forcing and Climate: Results From the Earth’s Radiation BudgetExperiment’, Science 243, 57–63.

Rind, D., and Overpeck, J.: 1993, ‘Hypothesized Causes of Decade-to-Century Climate Variations’,Quat. Sci. Rev. 12, 357–374.

Rossow, W. B., and Schiffer, R.: 1991, ‘International Satellite Cloud Climatology Project (ISCCP)cloud data products’, Bull. Amer. Meteor. 72, 2–20.

Rossow, W. B., and Cairns, B.: 1995, ‘Monitoring changes of clouds’, J. Climate 31, 305–347.Shindell, D., Rind, D., Balabhandran, N., Lean, J., and Lonergan, P.: 1999, ‘Solar Cycle Variability,

Ozone and Climate’, Science 284, 305.Stowe, L. L., Wellemayer, C. G., Eck, T. F., Yeh, H.Y.M., and the Nimbus-7 Team: 1988, ‘Nimbus-7

global cloud climatology, Part 1: Algorithms and validations’, J. Climate 1, 445–470.Svensmark, H., and Friis-Christensen, E.: 1997, ‘Variation of cosmic ray flux and global cloud

coverage - a missing link in solar-climate relationships’, J. Atm. Sol. Terr. Phys. 59, 1225–1232.Svensmark, H.: 1998, ‘Influence of cosmic rays on Earth’s climate’, Phys. Rev. Lett. 81, 5027–5030.Tinsley, B. A.: 1996, ‘Solar Wind Modulation of the Global Electric Circuit and the Apparent Ef-

fects on Cloud Microphysics, Latent Heat Release, and Tropospheric Dynamics’, J. Geomag.Geoelectr. 48, 165–175.

Weng, F., and Grody, N. C.: 1994, ‘Retrieval of Cloud Liquid Water Using the Special SensorMicrowave Imager’, J. Geophys. Res. 99, 25,535; Ferraro, R. R., Weng, F., Grody, N. C., andBasist, A.: 1996, ‘An Eight-Year (1987-1994) Time Series of Rainfall, Clouds, Water Vapor,Snow Cover, and Sea Ice Derived From SSM/I Measurements’, Bull. Am. Met. Soc. 77, 891–905.

Address for Offprints: Henrik Svensmark, Danish Space Research Institute, Juliane Maries Vej 30,DK-2100 Copenhagen Ø, Denmark; [email protected]


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